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Exergy Based Analysis for the Environmental Control and
Life Support Systems of the International Space Station
Kirk A. Clem 1, George J. Nelson, Ph.D. 2and Bryan L. Mesmer, Ph.D.3
University of Alabama in Huntsville, Huntsville, Alabama, 35899
Michael D. Watson, Ph.D.4 and Jay L. Perry5
NASA, George C. Marshall Space Flight Center, Huntsville, Alabama, 35812
When optimizing the performance of complex systems, a logical area for concern is im-
proving the efficiency of useful energy. The energy available for a system to perform work is
defined as a systemโs exergy content. Interactions between a systemโs subsystems and the sur-
rounding environment can be accounted for by understanding various subsystem exergy effi-
ciencies. Exergy balance of reactants and products, and enthalpies and entropies, can be used
to represent a chemical process. Heat transfer exergy represents heat loads, and flow exergy
represents system flows and filters. These elements allow for a system level exergy balance.
The exergy balance equations are developed for the subsystems of the Environmental Control
and Life Support (ECLS) system aboard the International Space Station (ISS). The use of
these equations with system information would allow for the calculation of the exergy
efficiency of the system, enabling comparisons of the ISS ECLS system to other systems as
well as allows for an integrated systems analysis for sytem optimization.
Nomenclature
ฮฑA = transfer coefficient
ฮt = change in time [s]
ฮX = change in exergy [J]
ฮท = efficiency
ฮทA = anode activation overpotential [V]
ฮทC = cathode activation overpotential [V]
ฮทI = overpotential due to interfacial resistance [V]
ฮทSPE = overpotential due to electrolyte membrane [V]
F = Faradayโs Constant [96485 C/mol]
h = specific enthalpy [J/kg]
H2 = Hydrogen
H2O = Water
i = current density [A/m2]
iA0 = anode exchange current density [A/m2]
iC0 = cathode exchange current density [A/m2]
I = total current [A]
LB = membrane thickness [m]
แน = mass flow rate [kg/s]
แน = molar flow rate [mol/s]
NIV = Nitrogen Isolation Valve
ฮฝe- = stoichiometric coefficient of electrons
1 Student, Department of Industrial and Systems Engineering and Engineering Management. 2 Assistant Professor, Department of Mechanical and Aerospace Engineering. 3 Assistant Professor, Department of Industrial and Systems Engineering and Engineering Management. 4 Flight Systems Engineer, Systems Engineering Management Office/EE11. 5 Lead Engineer, Environmental Control Systems, Space Systems Dept./ES62.
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OIV = Oxygen Isolation Valve
OPEN = discrete valve position (1 open, 0 closed)
O2 = oxygen
P = pressure [Pa]
Q = heat transfer [J]
Q = quality (% air)
Q = heat transfer rate [W]
R = gas constant (8.314 kJ/kmolยทK)
RI = interfacial resistance [ฮฉ]
s = entropy [J/kgยทK]
Sep = separator
ฯB = electrolyte conductivity [1/ฮฉยทm]
T = temperature
V = overall cell potential for electrolyzer [V]
V0 = Nernst Potential [V]
Vol = volume [m3]
W = work [J]
แบ = power [W]
WRM = water recovery and management
X = exergy [J]
แบ = rate of exergy transfer associated with flow [W]
แบdes = rate of exergy destroyed [W]
I. Introduction
HE subsystems that make up the Environmental Control and Life Support (ECLS) system aboard the International
Space Station (ISS) perform multiple functions. Cabin atmospheric regulation and revitalization, waste gas re-
moval, oxygen generation, temperature and humidity control, urine processing, and water recovery are a mere handful
of system processes that may be broken down and analyzed individually. The maximum work output attainable by
each subsystem in relation to the dead state (i.e., reference state) of a specified operating environment may be ex-
pressed as a particular subsystemโs exergy. For the purposes of this study, exergy may be thought of as a โcommon
currencyโ for a system to perform work. Exergy provides a system-wide attribute that can be used to measure perfor-
mance of all subsystems, and relates subsystem products (outputs) to subsystem reactants (inputs).1 An exergy balance
describes the process that reactants and products undergo related to their enthalpies and entropies. Forms of exergy
examined in this paper include heat transfer exergy, fluid flow exergy, and electrical work exergy. In this paper the
exergy balance equation incorporating an exergy analysis of all subsystems of the ECLS is developed. These balances
can be used to determine the efficiency of the system as a whole.
II. Overview of ECLS for Purposes Beyond Low Earth Orbit
As crewed space exploration
objectives expand beyond low-
Earth orbit, the ECLS system must
safely enable nearly autonomous
missions within an energy and re-
source efficient architecture. The
ECLS systems of the future must be
capable of operating for 500 to
1200 days with no Earth-based lo-
gistics resupply and be highly relia-
ble and maintainable. The ECLS
systems of the future, however, will
possess many functional similari-
ties to the system deployed aboard
the ISS; therefore, studying the ISS
T
Figure 1. Functions provided by the ECLS system.
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ECLS system provides important insight. The principal functions that ECLS systems must provide for long duration
missions are presented by Fig. 1.
The interfaces between these functions, illustrated by Fig. 2, will likely be common between the ISS and explora-
tion missions. As such, the ISS ECLS not only serves as the technical basis for exploration ECLS system architectures,
but also serves as a unique developmental laboratory to test candidate technologies and system performance evaluation
techniques that can help improve the ECLS system design over the ISS state-of-the-art. Applying exergy metrics to
ECLS systems, using the ISS ECLS system architecture as the starting model, may yield a new tool for optimizing the
system beyond the traditional mass, power, and volume objectives.
III. Background
A. Exergy
Exergy, a measure of work potential, is derived from the first and second laws of thermodynamics. The first law
of thermodynamics states that there is a constant quantity of energy in the universe and asserts that energy is neither
created nor destroyed but may be transformed. Thus the first law, a law of conservation, allows the engineer to track
energy through a process. The second law of thermodynamics is concerned with the degradation of energy during a
process. The relationship between first and second law analyses is seen in Fig. 3. Entropy generation and the lost
ability to perform work suggest that the energy efficiency of a system may be improved.2
Corresponding to exergy, the maximum useful work that can be produced as a system performs a process between
two specified states is identified as reversible
work. In the case of reversible work, a system
can rejuvenate itself to its original state in an im-
promptu fashion. This phenomenon does not oc-
cur in nature as real processes are irreversible.
Additional energy is required to overcome
losses and restore a system to its initial regime,
and entropy is generated.
Many different fields have shown that there
is much value to be gained by studying how ex-
ergy across a particular system changes through
Figure 2. Functional interactions within an ECLS system architecture.
Figure 3. First and second laws of thermodynamics.
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process. Production processes of many kinds can be modeled using exergy principles to describe resource and waste
energy.3 These principles may be useful for other studies such as evaluating costs of different forms of energy4 such
as power generation,5-9 and for studies of hypersonic aircraft.10 Camberos and Moorhouse have shown the use of
exergy as a metric for modeling, constructing, and modification of aerospace vehicles.11 Doty, et al., has applied
exergy to the early stages of turbine design enhancement.12 The work in this paper is in association with the NASA
Systems Engineering Research Consortium. In previous work, the consortium has examined the use of exergy metrics
in rocket systems.13-14 This previous work has shown an improved understanding of efficiencies when dealing with
complex engineered systems. Future work with rocket systems will be examining the use of exergy in the optimization
of rocket staging.
B. Exergy Balance
The general exergy rate balance for an open system, or control volume, is shown in Eq. 1.
๐๐
๐๐ก= โ (1 โ
๐0
๐๐) ๏ฟฝ๏ฟฝ๐๐ + (๏ฟฝ๏ฟฝ๐๐ โ ๐0
๐๐๐๐
๐๐กโ ๏ฟฝ๏ฟฝ๐๐ข๐ก) + โ ๏ฟฝ๏ฟฝ๐๐๐๐ โ โ ๏ฟฝ๏ฟฝ๐๐๐ข๐ก๐ โ ๏ฟฝ๏ฟฝ๐๐๐ (1)
On the right hand side of Eq. 1, T0 represents the temperature of an environment, Q is the rate of heat transfer in and
out, W is power in and out, P0 is the pressure of the environment, Vol is the volume of the initial and final states, and
๐ represents a rate of exergy transfer by the fluid shown in Eq. 2. The exergy transfer rates are associated with either
mass flow into the system, mass flow out of the system, or exergy destruction associated with irreversibility in the
system. For the ECLS system, the cabin is taken as the reference environment unless otherwise stated.
๐ = โ โ ๐0๐ +๐2
2+ ๐๐ง (2)
On the right hand side of Eq. 2, h is the enthalpy of the fluid stream, T0 is the reference temperature (i.e., cabin
temperature), s is the entropy of the fluid stream, v is the velocity of the fluid stream, g is the gravitational constant
(at the reference orbit altitude), and z is the change in height.
The left hand side of Eq. 1 represents the exergy stored within the system. Under steady state operation, this storage
term, and the change in system volume with time, are eliminated. This condition gives the following steady state
exergy balance for an open system.
๏ฟฝ๏ฟฝ๐๐๐ = โ (1 โ๐0
๐๐) ๏ฟฝ๏ฟฝ๐๐ + (๏ฟฝ๏ฟฝ๐๐ โ ๏ฟฝ๏ฟฝ๐๐ข๐ก) + โ ๏ฟฝ๏ฟฝ๐๐๐๐ โ โ ๏ฟฝ๏ฟฝ๐๐๐ข๐ก๐ (3)
For a real system operating in the presence of irreversibility this exergy destruction will be positive. Eq. 3 indicates
that under steady state operation the rate of exergy destroyed is balanced by the exergy transferred through work, heat
transfer, and fluid flow. Under a limited set of inlet and exit conditions, an ideal case may be constructed such that the
right hand side of the equation yields แบdes = 0. This condition is associated with reversible operation of the system.
C. Exergy Efficiency
Under ideal, reversible operation no exergy is destroyed. The exergy balance in Eq. 3 can be recast to define the
reverisble power production, Eq. 4. Here it is important to note that this reversible power is treated as a net power out
of the system.
๏ฟฝ๏ฟฝ๐๐๐ฃ = โ (1 โ๐๐
๐๐) ๏ฟฝ๏ฟฝ๐๐ + โ ๏ฟฝ๏ฟฝ๐๐๐๐ โ โ ๏ฟฝ๏ฟฝ๐๐๐ข๐ก๐ (4)
Comparing the actual power produced in the presence of exergy destruction to reversible power production pro-
vides a means for defining the system exergy efficiency. The overall exergy efficiency for any system is given by Eq.
5.
๐๐๐ฅ๐๐๐๐ฆ =๐ธ๐ฅ๐๐๐๐ฆ ๐ ๐๐๐๐ฃ๐๐๐๐
๐ธ๐ฅ๐๐๐๐ฆ ๐ธ๐ฅ๐๐๐๐๐๐= 1 โ
๐ธ๐ฅ๐๐๐๐ฆ ๐ท๐๐ ๐ก๐๐๐ฆ๐๐
๐ธ๐ฅ๐๐๐๐ฆ ๐ธ๐ฅ๐๐๐๐๐๐ (5)
In Eq. 5, แบactual is associated with exergy that is effectively used by the real system to generate power and แบdes is
associated with the loss term observed in the exergy balance equation. As defined in Eq. 4, the reversible power
American Institute of Aeronautics and Astronautics
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represents the maximum attainable power for a given set of environmental conditions, which corresponds to the ab-
sence of irreversibility during operation. The exergy efficiency equation allows for the ranking of the various subsys-
tems from most thermodynamically efficient to least thermodynamically efficient. The system-wide metric of exergy
efficiency is applicable to all engineering disciplines. For a given system analyzed on a rate basis, the maximum
exergy efficiency corresponds to the minimum exergy destruction rate.
The ECLS system aboard the ISS is composed of various subsystems that perform functions such as atmospheric
revitalization, waste management, and temperature and humidity control.15 This paper will develop the exergy balance
equations for the following subsystemsโAtmosphere Control and Supply (ACS), Atmosphere Revitalization (AR),
Temperature and Humidity Control (THC), and Waste Recovery and Management (WRM). The objectives for this
work are to simplify known exergy equations used to model the subsystems and add the product and reactant terms
for the chemical processes. When performing this process, it will be important to note that for a serial string of pro-
cessing stages, the intermittent values cancel whereby the output of one stage is the input to the next stage. Thus, the
input to the subsystem and the output of the subsystem will be known with intermediate steps cancelling. To under-
stand the interactions in the subsystem, a model incorporating exergy is created in Matlabยฎ. The Matlabยฎ models
enable future analysis of the subsystems with real-world data in terms of efficiency and enable optimization of the
subsystem in future work. The following presents the exergy models developed for the ISS ECLS system.
IV. Exergy Analysis of ECLS Subsystems
This paper first presents a detailed examination of an assembly within the AR subsystem to illustrate the detailed
process for developing exergy models. This assembly, the Oxygen Generation Assembly (OGA), is highly detailed to
provide the logical flow used in forming the exergy balances for the other subsystems examined. Following the de-
tailed treatment of the OGA, each ECLS subsystem is described and their exergy balance equations are developed.
A. Exergy Analysis of the Oxygen Generation Assembly
The OGA is an assembly within the ARS. Equation 6 demonstrates the exergy relationships involved for the
OGA.
(6)
The term on the left hand side of the equation
represents the rate of exergy destruction
throughout electrolysis. The first term on the
right hand side of the equation represents the
power input from the water electrolyzer. The
second term represents the enthalpy-entropy
difference of oxygen (O2) multiplied by the ox-
ygen mass flow rate out of the O2 separator. The
third term represent the enthalpy-entropy dif-
ference of hydrogen (H2) multiplied by the hy-
drogen mass flow rate out of the H2 separator.
The fourth term represents the enthalpy-entropy
difference of water multiplied by the mass flow
rate of water entering the electrolyzer.
A simplified schematic of the OGA is shown in Fig. 4. Water enters the assembly from the ISS potable
water bus through the Water Assembly On-orbit Replacement Unit (ORU). An Inlet Deionizing (DI) Bed serves
as an iodine remover and as a coalescer for the removal of gas bubbles from the feed water. Should gas bubbles
be detected past the DI bed by a downstream gas sensor, a three-way valve sends the feed water to a waste
water bus. This prevents any O2 that may be present in the feed water from mixing with any H2. Electrolysis
takes place in a Hydrogen Dome ORU where water molecules are disassociated into O2 and H2 gas. A Rotary
Separator Accumulator (RSA) is also located within the hydrogen dome. The RSA removes H2 that leaves the
cathode from water which is recirculated by a positive displacement pump ORU. The removed H2 is then sent
to a Sabatier-based carbon dioxide (CO2) reduction process or is vented to space. The Power Supply Module
(PSM) ORU powers the OGA electrolysis cell stack providing 10-55 amps of current during process mode, and
1.0 amps during standby mode which produces a 2% O2 rate. This power supply allows for a nominal O2
Figure 4. Simplified OGA process schematic.
แบ๐ซ๐๐,๐ถ๐ฎ๐จ = แบ๐๐๐๐๐๐๐๐๐๐๐๐ โ ๏ฟฝ๏ฟฝ๐ถ๐,๐ถ๐ ๐บ๐๐(โ๐2,๐๐ข๐ก โ ๐๐๐๐๐๐๐ ๐2,๐๐ข๐ก) โ ๏ฟฝ๏ฟฝ๐ฏ๐,๐ฏ๐ ๐บ๐๐(โ๐ป2,๐๐ข๐ก โ ๐๐๐๐๐๐๐ ๐ป2,๐๐ข๐ก)
+๏ฟฝ๏ฟฝ๐ป2๐,๐๐ ๐(โ๐ป2๐,๐๐ ๐ โ ๐๐๐๐๐๐๐ ๐ป2๐,๐๐ ๐)
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production rate of 5.4 kg/day which is enough to support four crewmembers. A maximum oxygen production
rate of 9.2 kg/day may be achieved which provides enough oxygen to sustain seven crewmembers.16
A goal for conducting thermodynamic analysis of O2 and H2 generating electrolyzers is to identify key losses so
that system performance may be optimized. Water electrolyzers run in an exothermic mode. Heat production that
occurs within the electrolytic cells is caused by irreversible losses. These losses stem from overpotentials which exceed
the thermal energy demand. If electrical energy is the dominant energy input, exergy efficiency will be almost identical
to the systemโs energy efficiency. Meng et.al has identified parameters that would improve the exergy efficiency of
water electrolyzers including reducing and controlling operating temperature, increasing current density, increasing
electrolyte thickness, and decreasing electrode catalytic activity.17
Valverde et al.18 proposed a simple model for low-pressure water electrolyzers. The model provides enough
precision for simulation of electrolchemical and thermal output flow behaviors. Choi et al. performed water
electrolysis analysis using a simple model based on Butler-Volmer kinetics for electrodes and transport resistance in
polymer electrolyte. The model demonstrates the behavior of applied thermal voltage and current density in terms of
Nernst potential, exchange current desities, and polymer elctrolyte conductivity. Analyzed with in the model are
overpotential and resistances at the anode and cathode, as well as overpotential due to ohmic losses.19
During electrolysis, water and O2 escape from the anode of the electrolytic cell, and H2 is produced from the cellโs
cathode. Their corrsponding mass balances are written as follows:
แนH2O,in - แนH2O, out =๐๐ด
2๐น (7)
แนH2,in - แนH2, out = โ๐๐ด
2๐น (8)
แนO2,in - แนO2, out = โ๐๐ด
4๐น (9)
The overall electrochemical reaction at the anode may be obtained by encorporating the Butler-Volmer expression:19
๐ = ๐๐ด0[exp (๐ผ๐ด ๐๐โ โ๐น๐๐ด
๐ ๐) โ exp (โ
(1โ๐ผ๐ด)๐๐โ๐น๐๐ด
๐ ๐)] (10)
If the effective transfer coefficient is assumed to be 0.5, and the stoichiometric coefficient of electrons in the anode is
assumed to be 2, then the anode overpotential takes the form as follows:
ฮทA = ๐ ๐
๐น๐ ๐๐โโ1(
๐
2๐๐ด0) (11)
If the effective transfer coefficient, ฮฑA, is assumed to be 0.5, but now the stoichiometric coefficient of electrons in the
cathode is assumed to be -2, then the cathode overpotential takes the form as follows:
๐๐ถ = โ๐ ๐
๐น๐ ๐๐โโ1(
๐
2๐๐ด0) (12)
For determination of the electrochemical potential of the electrolysis cell the Nernst potential, anode overpotential,
cathode overpotential, overpotential due to membrane, and interfacial overpotential due to resistance must be deter-
mined. The overall cell potential takes the following form:
V = V0 + ฮทA โ ฮทC + ฮทSPE + ฮทI (13)
The Nernst potential is empirically given as:
V0 = 1.23 โ 0.9E-3(T โ 298) + 2.3 ๐ ๐
4๐นlog (๐H2
2Po2) (14)
Overpotential due to membrane resistance is given as follows:
๐๐๐๐ธ = (๐ฟ๐ต
๐๐ต) ๐ (15)
Interfacial overpotential is calculated as follows:
๐ I = RI I (16)
Here, RI is the interfacial resistance of the electrolyzer and I is the total applied current. Once the overall
required cell potential is determined, the required power is obtained:
แบele = VI (17)
After defining the required power required for electrolysis, the rate of exergy destroyed by the oxygen genera-
tion process may be determined as follows:
(18) แบDES,OGA = แบELE โ ๏ฟฝ๏ฟฝ๐ถ๐,๐ถ๐ ๐บ๐๐(โ๐2,๐๐ข๐ก โ ๐๐๐๐๐๐๐ ๐2,๐๐ข๐ก) โ ๏ฟฝ๏ฟฝ๐ฏ๐,๐ฏ๐ ๐บ๐๐(โ๐ป2,๐๐ข๐ก โ ๐๐๐๐๐๐๐ ๐ป2,๐๐ข๐ก)
+๏ฟฝ๏ฟฝ๐ป2๐,๐๐ ๐(โ๐ป2๐,๐๐ ๐ โ ๐๐๐๐๐๐๐ ๐ป2๐,๐๐ ๐)
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B. Exergy Analysis of the Atmosphere Revitalization Subsystem
The ARS removes CO2 and other trace gases produced by crewmembers and equipment offgassing that are
potentially harmful to their health. The ARS also monitors the space stationโs atmosphere for major cons titu-
entsโO2, N2, CO2, methane (CH4), and water vapor. The major assemblies within the ARS are the Carbon
Dioxide Removal Assembly (CDRA), the Oxygen Generation Assembly (OGA), the Trace Contaminant Con-
trol Subassembly (TCCS), and the Major Constituent Analyzer (MCA).
1. Carbon Dioxide Removal Assembly
The CDRA is a 4-bed molecular sieve (4BMS) process which removes CO2 from the cabin atmosphere via ad-
sorption using two zeolite 5A molecular sieve pellet beds. The two beds operate in cycles allowing one bed to regen-
erate by desorption of CO2 while the other bed cycles through a CO2 adsorption process. This provides continuous
operation for the removal of CO2. Water vapor is first adsorbed from the inlet gases onto beds of zeolite 13X and silica
gel. The inlet duct to the CDRA interfaces downstream of a condensing heat exchanger where moisture is removed
from the cabin atmosphere by condensation. Here, bulk moisture is removed, reducing the water load presented to the
desiccant beds in the CDRA. Components within the CDRA include desiccant beds/CO2 adsorbent beds, air selector
and check valves, a pre-cooler, blower assembly, and an air save bump.20 The CDRA process may be analyzed holis-
tically from an exergy perspective as follows:
(19)
2. Oxygen Generation Assembly
The exergy analysis of the OGA was discussed previously in Section B.
3. Trace Contaminant Control Subassembly
The TCCS removes and disposes of gaseous contaminants. It is composed of a charcoal bed, flow meter, blower,
catalytic oxidizer, and a sorbent bed. The charcoal bed contains a disposable activated carbon for removing compounds
with larger molecular weights. The blower within the TCCS controls system airflow, and is designed similar to the
blower in the CDRA. The catalytic oxidizer assembly contains palladium (Pd) on an alumina (Al2O3) catalyst which
converts carbon monoxide (CO), CH4, H2, and other compounds with low molecular weights that the charcoal bed is
unable to absorb into CO2, water, and other acceptable compounds. The sorbent bed contains granular lithium hydrox-
ide (LiOH) to remove any undesirable acidic byproducts of catalytic oxidation such as hydrogen chloride (HCl), chlo-
rine (Cl2), fluorine (F2), nitrogen dioxide (NO2), and sulfur dioxide (SO2).20 The TCCS is analyzed from an exergy
perspective as follows:
(20)
๏ฟฝ๏ฟฝ๐๐๐ ,4๐ต๐๐ = แบ๐ช๐ถ๐ ๐ฝ๐๐๐๐ ๐บ๐๐๐๐๐๐๐ + แบ๐ช๐ถ๐ ๐๐๐๐๐๐๐/๐๐๐๐๐๐๐๐๐๐๐ โ [๏ฟฝ๏ฟฝ๐๐๐,๐ ๐๐๐๐๐๐๐๐๐ ๐๐๐(๐๐๐๐,๐ ๐๐๐๐๐๐๐๐ ๐๐๐ โ
๐๐๐๐๐๐๐๐๐๐,๐ ๐๐๐๐๐๐๐๐ ๐๐๐) โ ๏ฟฝ๏ฟฝ๐๐๐,๐ช๐ซ๐น๐จ ๐๐(๐๐๐๐,๐ช๐ซ๐น๐จ ๐๐ โ ๐๐๐๐๐๐๐๐๐๐,๐ช๐ซ๐น๐จ ๐๐) โ ๏ฟฝ๏ฟฝ๐ ๐๐๐๐๐๐ ,๐ฏ๐๐ถ ( ๐ฝ๐ฏ๐๐ถ,๐ช๐ซ๐น๐จ
๐
๐) โ
๏ฟฝ๏ฟฝ๐ ๐๐๐๐๐๐๐ ,๐ฏ๐๐ถ ( ๐ฝ๐ฏ๐๐ถ,๐ช๐ซ๐น๐จ๐
๐
๐)]
๐ ๐๐๐๐๐๐๐๐๐+ [แบ๐ช๐ซ๐น๐จ ๐ฉ๐๐๐๐๐ โ (๐ โ
๐ป๐๐๐๐๐
๐ป๐ฉ๐๐๐๐๐)๏ฟฝ๏ฟฝ๐ถ๐ท๐ ๐ด ๐ต๐๐๐ค๐๐]
๐ฉ๐๐๐๐๐+
[โ (๐๐๐๐๐๐โ๐๐๐๐๐๐๐๐ก
๐๐๐๐๐๐๐๐ก) ๏ฟฝ๏ฟฝ๐ถ๐ท๐ ๐ด ๐๐๐๐๐๐๐๐๐]
๐๐๐๐๐๐๐๐๐+ [๏ฟฝ๏ฟฝ๐ ๐๐๐๐๐๐ ,๐ช๐ถ๐ ๐บ๐๐๐๐๐๐ (
๐ฝ๐ช๐ถ๐,๐ช๐ซ๐น๐จ๐
๐)]
๐ช๐ถ๐ ๐จ๐ ๐๐๐๐๐๐๐๐๐+
๐๐๐ธ๐๐ด๐ [แบ๐จ๐บ ๐๐๐๐ โ (๐ โ๐ป๐๐๐๐๐
๐ป๐จ๐บ ๐ท๐๐๐)๏ฟฝ๏ฟฝ๐จ๐บ ๐๐๐๐ โ ๏ฟฝ๏ฟฝ๐๐๐,๐๐๐๐๐๐๐ก ๐๐ข๐ก ((๐๐๐๐,๐ด๐ ๐๐ข๐๐ ๐๐ข๐ก โ
๐๐๐๐๐๐๐๐๐๐,๐ด๐ ๐๐ข๐๐ ๐๐ข๐ก) โ (๐๐๐๐,๐ด๐ ๐๐ข๐๐ ๐๐ โ ๐๐๐๐๐๐๐๐๐๐,๐ด๐ ๐๐ข๐๐ ๐๐))]๐ด๐๐ ๐๐๐ฃ๐
+ [แบ๐ช๐ถ๐ ๐บ๐๐๐๐๐๐ ๐ฏ๐๐๐๐๐ โ
๐๐๐ธ๐๐ถ๐2 ๐๐๐๐ก๏ฟฝ๏ฟฝ๐ ๐๐๐๐๐๐ ,๐ช๐ถ๐ ๐๐๐๐๐๐๐(๐๐๐๐,๐ช๐ถ๐ ๐๐๐๐๐๐๐ โ ๐๐๐๐๐๐๐๐๐๐,๐ช๐ถ๐ ๐๐๐๐๐๐๐)]๐ช๐ถ๐ ๐ ๐๐๐๐๐๐๐๐๐
แบdes,TCCS = [๏ฟฝ๏ฟฝ๐๐๐,๐ป๐ช๐ช๐บ ๐ช๐ฉ ๐ฝ๐๐๐,๐ช๐ฉ ๐๐๐
๐
๐+ ๏ฟฝ๏ฟฝ๐ป๐ช๐ช๐บ ๐ ๐๐๐๐๐๐ ,๐๐๐๐๐๐ (
๐ฝ๐๐๐,๐ช๐ฉ ๐๐๐
๐) โ ๏ฟฝ๏ฟฝ๐๐๐,๐ป๐ช๐ช๐บ ๐๐
๐ฝ๐๐๐,๐ช๐ฉ ๐๐๐
๐]
๐ช๐ฉ+
[แบ๐๐ถ๐ถ๐ ๐ต๐๐๐ค๐๐ โ (๐ โ๐ป๐๐๐๐๐
๐ป๐ฉ๐๐๐๐๐) ๏ฟฝ๏ฟฝ
๐๐ถ๐ถ๐ ๐ต๐๐๐ค๐๐ โ ๏ฟฝ๏ฟฝ๐๐๐,๐๐ถ๐ถ๐ ๐ถ๐ต ((๐๐๐๐,๐๐ถ๐ถ๐ ๐ต๐๐๐ค๐๐ ๐๐ข๐ก โ
๐๐๐๐๐๐๐๐๐๐,๐๐ถ๐ถ๐ ๐ต๐๐๐ค๐๐ ๐๐ข๐ก) โ (๐๐๐๐,๐๐ถ๐ถ๐ ๐ต๐๐๐ค๐๐ ๐๐ โ ๐๐๐๐๐๐๐๐๐๐,๐๐ถ๐ถ๐ ๐ต๐๐๐ค๐๐ ๐๐))]๐ฉ๐๐๐๐๐
+
{[๏ฟฝ๏ฟฝ๐๐๐,๐๐ถ๐ถ๐ ๐ฟ๐๐๐ป(โ๐๐๐,๐๐ถ๐ถ๐ ๐ฟ๐๐๐ป โ ๐๐๐๐๐๐๐ ๐๐๐,๐๐ถ๐ถ๐ ๐ฟ๐๐๐ป) + ๏ฟฝ๏ฟฝ๐ณ๐๐ถ๐ฏ ๐ ๐๐๐๐๐๐ ( ๐๐๐๐,๐๐ถ๐ถ๐ โ๐๐๐ก๐๐
2
2)]
๐ฟ๐๐๐ป+
[แบ๐ฏ๐ป๐ช๐ถ ๐๐๐๐๐๐ โ ๏ฟฝ๏ฟฝ๐๐๐,๐๐๐๐ค๐๐ ๐ป๐๐ถ๐(โ๐๐๐,๐๐ถ๐ถ๐ ๐ต๐๐๐ค๐๐ ๐๐ข๐ก โ ๐๐๐๐๐๐๐ ๐๐๐,๐๐ถ๐ถ๐ ๐ต๐๐๐ค๐๐ ๐๐ข๐ก)]โ๐๐๐ก๐๐
+
[โ๏ฟฝ๏ฟฝ๐๐๐,๐๐๐๐ค๐๐ ๐๐๐๐๐๐๐(โ๐๐๐,๐๐ถ๐ถ๐ ๐๐๐๐๐๐๐ ๐๐ข๐ก โ ๐๐๐๐๐๐๐ ๐๐๐,๐๐ถ๐ถ๐ ๐๐๐๐๐๐๐ ๐๐ข๐ก) โ
๏ฟฝ๏ฟฝ๐๐๐,๐๐๐๐ค๐๐ ๐๐๐๐๐๐๐(โ๐๐๐,๐๐ถ๐ถ๐ ๐ต๐๐๐ค๐๐ ๐๐ข๐ก โ ๐๐๐๐๐๐๐ ๐๐๐,๐๐ถ๐ถ๐ ๐ต๐๐๐ค๐๐ ๐๐ข๐ก)]๐๐๐๐๐๐๐
}๐ป๐๐ถ๐
American Institute of Aeronautics and Astronautics
8
4. Major Constituent Analyzer
The MCA functions by drawing samples past a single-focusing magnetic sector mass spectrometer where
drawn gas molecules are ionized by an ion source. The ionized molecules are then accelerated by an electron
field and move into a shaped magnetic field where they may be dispersed by molecular weight. Dispersed ion
beams are then focused into Faraday current collectors by resolving slits. Collected currents are proportional
to partial pressures. Molecules that are not collected are absorbed by an ion pump. Air that is not able to enter
the mass spectrometer is delivered to the ARS rack by pump.20 Exergy destroyed from the MCA is modeled as
follows:
(21)
C. Exergy Analysis of the Atmosphere Control and Supply System
The ACS subsystem performs multiple functions such as cabin pressure control, atmospheric composition
control, O2/N2 partial pressure control, provides over- and under-pressure relief of any particular module, pro-
vides the ability to evacuate the atmosphere of any individual module, manual pressure equalization capability
at module interfaces, and allows for internal distribution of O2 and N2 to ISS systems, crew, and payload inter-
faces at desired temperatures, pressures and flow rates. The ACS subsystem consists of the following compo-
nents that are analyzed from an exergy perspective: pressure control assembly (PCA), O2 and N2 storage and
distribution, manual pressure equalization valve (MPEV), N2 interface assembly (NIA), and airlock air save
pump package.20 The ACS subsystem as a whole may be analyzed from an exergy perspective as follows:
(22)
1. Pressure Control Assembly
The PCA controls and monitors the total pressure within the module by controlling oxygen and nitrogen partial
pressures. The PCA allows for controlled venting to space of on board gasses, and provides controlled re-pressuriza-
tion capability. The first component analyzed from the PCA is the mixing diffuser. Exergy destroyed within the mixing
diffuser is defined by the changes in enthalpy, entropy, and kinetic energy. The mixing diffuser is fed by a mixing
chamber which allows for gases to enter the cabin.
(23)
Gas flow through restrictors and interface valves contained within the PCA is next analyzed from an exergy perspec-
tive. Work performed when opening and closing restrictors and interface valves is a small change in kinetic energy,
and the electrical power supplied to actuate the valves.
แบDES,DIFFUSER = ๏ฟฝ๏ฟฝ๐ถ๐ ((๐๐2,๐๐ โ ๐๐๐๐,๐๐๐๐๐) โ ๐๐๐๐๐๐(๐๐2,๐๐ โ ๐๐๐๐,๐๐๐๐๐) + ๐ฝ๐ถ๐,๐ถ๐ฐ๐ฝ
๐
๐)
+๏ฟฝ๏ฟฝ๐ต๐ ((๐๐2,๐๐ โ ๐๐๐๐,๐๐๐๐๐) โ ๐๐๐๐๐๐(๐๐2,๐๐ โ ๐๐๐๐,๐๐๐๐๐) + ๐ฝ๐ต๐,๐ต๐ฐ๐ฝ
๐
๐)
แบdes,MCA = [แบ๐ด๐ช๐จ ๐ด๐บ โ (๐๐๐๐,๐๐ถ๐ด ๐๐ ๐ฃ๐๐๐ก โ ๐๐ ๐๐๐๐๐๐๐๐,๐๐ถ๐ด ๐๐ถ๐ด ๐ฃ๐๐๐ก) + (๐๐๐๐,๐๐ถ๐ด ๐๐ ๐๐ โ
๐๐๐๐๐๐๐๐๐๐,๐๐ถ๐ด ๐๐ ๐๐)]๐ด๐ช๐จ ๐ด๐บ
+ [แบ๐ด๐ช๐จ ๐๐๐๐ โ (๐ โ๐ป๐๐๐๐๐
๐ป๐ฉ๐๐๐๐๐)
๏ฟฝ๏ฟฝ๐บ๐ถ ๐๐ข๐๐ โ
๏ฟฝ๏ฟฝ๐๐๐,๐ด๐ช๐จ ๐๐๐๐ ๐๐ ((๐๐๐๐,๐๐ถ๐ด ๐๐ข๐๐ ๐๐ข๐ก โ ๐๐๐๐๐๐๐๐๐๐,๐๐ถ๐ด ๐๐ข๐๐ ๐๐ข๐ก) โ (๐๐๐๐,๐๐ถ๐ด ๐๐ข๐๐ ๐๐ โ
๐๐๐๐๐๐๐๐๐๐,๐๐ถ๐ด ๐๐ข๐๐ ๐๐))]๐ด๐ช๐จ ๐๐๐๐
+ แบ๐ด๐ช๐จ ๐๐๐๐๐๐ + [๐ท๐๐๐๐๐(๐๐๐๐๐๐ ๐๐๐๐ ๐๐๐๐ โ ๐๐๐๐๐๐๐๐ ๐๐๐๐ ) โ
๐๐๐๐๐๐(๐๐๐๐ ๐๐๐๐ ๐๐๐๐ โ ๐๐๐๐,๐๐๐๐๐)]๐ฅ๐
๐ฅ๐ก ๐ฝ๐ฎ๐จ
แบdes,ACS = [(๐ท๐๐๐๐๐(๐๐๐๐2 ๐๐๐๐ ๐๐๐๐ โ ๐๐๐๐๐๐๐๐ ๐๐๐๐ ) โ ๐๐๐๐๐๐(๐๐2 ๐๐๐๐ ๐๐๐๐ โ ๐๐๐๐,๐๐๐๐๐)) +
(๐ท๐๐๐๐๐(๐๐๐๐2 ๐๐๐๐ ๐๐๐๐ โ ๐๐๐๐๐๐๐๐ ๐๐๐๐ ) โ ๐๐๐๐๐๐(๐๐2 ๐๐๐๐ ๐๐๐๐ โ ๐๐๐๐,๐๐๐๐๐))]๐ฅ๐
๐ฅ๐ก ๐บ๐๐๐๐๐๐โ
{[๏ฟฝ๏ฟฝ๐ถ๐ ( ๐ฝ๐ถ๐,๐๐
๐
๐) + แบ๐ถ๐ฐ๐ฝ + ๏ฟฝ๏ฟฝ๐ต๐ (
๐ฝ๐ต๐,๐๐๐
๐) + แบ๐ต๐ฐ๐ฝ]
๐น๐ฝ+ [๏ฟฝ๏ฟฝ๐ถ๐ ((๐๐2,๐๐ โ ๐๐๐๐,๐๐๐๐๐) โ ๐๐๐๐๐๐(๐๐2,๐๐ โ
๐๐๐๐,๐๐๐๐๐)) + ๏ฟฝ๏ฟฝ๐ต๐ ((๐๐2,๐๐ โ ๐๐๐๐,๐๐๐๐๐) โ ๐๐๐๐๐๐(๐๐2,๐๐ โ ๐๐๐๐,๐๐๐๐๐))]๐ด๐๐ ๐ซ๐๐๐
} โ
{[๐ถ๐ท๐ฌ๐ต๐ด๐ท๐ฌ๐ฝ (๏ฟฝ๏ฟฝ๐๐๐,๐ด๐ท๐ฌ๐ฝ(๐๐๐๐,๐๐๐๐๐1 โ ๐๐๐๐,๐๐๐๐๐ ๐๐๐๐๐๐๐))]๐ธ๐๐ข๐๐๐๐ง๐๐ก๐๐๐
+
[๐ถ๐ท๐ฌ๐ต๐ฝ๐น๐ฐ๐ฝ๐ถ๐ท๐ฌ๐ต๐ฝ๐น๐ช๐ฝ (๏ฟฝ๏ฟฝ๐๐๐,๐ฝ๐น๐ฝ ((๐๐๐๐,๐๐๐๐๐ โ ๐๐๐๐,๐ ๐๐๐๐) โ ๐๐ ๐๐๐๐(๐๐๐๐,๐๐๐๐๐ โ ๐๐๐๐,๐ ๐๐๐๐) +
๐ฝ๐๐๐,๐๐๐๐๐
๐) + แบ๐ฝ๐น๐ฐ๐ฝ + แบ๐ฝ๐น๐ช๐ฝ)]
๐๐๐๐ก
}
๐๐๐๐ ๐ ๐๐
+ แบ๐ฌ๐๐๐ ๐บ๐๐๐๐ ๐๐
American Institute of Aeronautics and Astronautics
9
แบDES,RESTRICTOR = ๏ฟฝ๏ฟฝ๐ถ๐ ( ๐ฝ๐ถ๐,๐๐
๐
๐โ
๐ฝ๐ถ๐,๐ถ๐ฐ๐ฝ๐
๐) + แบ๐ถ๐ฐ๐ฝ + ๏ฟฝ๏ฟฝ๐ต๐ (
๐ฝ๐ต๐,๐๐๐
๐โ
๐ฝ๐ต๐,๐ต๐ฐ๐ฝ๐
๐) + แบ๐ต๐ฐ๐ฝ (24)
Vent Relief Valves (VRV) may be similarly analyzed from an exergy perspective as follows:
(25)
2. Oxygen and Nitrogen Storage and Distribution
The O2 and N2 storage and distribution systems consist of storage tanks, interface valves, and distribution lines
which allow for controlled delivery of pressurized oxygen and nitrogen to required destinations within the spacecraft.
Exergy stored within the N2 and O2 storage tanks may be modeled as follows:
๐ฟ๐๐๐๐๐๐ ๐๐๐๐๐๐๐๐ = [(๐ท๐๐๐๐๐(๐๐๐๐2 ๐๐๐๐ ๐๐๐๐ โ ๐๐๐๐๐๐๐๐ ๐๐๐๐ ) โ ๐๐๐๐๐๐(๐๐2 ๐๐๐๐ ๐๐๐๐ โ ๐๐๐๐,๐๐๐๐๐))]๐บ๐๐๐๐๐๐
(26)
๐ฟ๐๐๐๐๐๐ ๐๐๐๐๐๐ = [(๐ท๐๐๐๐๐(๐๐๐๐2 ๐๐๐๐ ๐๐๐๐ โ ๐๐๐๐๐๐๐๐ ๐๐๐๐ ) โ ๐๐๐๐๐๐(๐๐2 ๐๐๐๐ ๐๐๐๐ โ ๐๐๐๐,๐๐๐๐๐))]๐บ๐๐๐๐๐๐
(27)
3. Manual Pressure Equalization Valve
The MPEVs, located in all hatches, are used when equalizing pressure in two adjacent pressurized modules. This
equalization is performed prior to opening the hatches that separate the modules. Other applications for MPEVs in-
clude collecting atmosphere samples from, and to measure pressure within a module prior to opening a hatch. The rate
of exergy destroyed when operating the MPEV occurs as follows:
แบDES,MPEV = ๐ถ๐ท๐ฌ๐ต๐ด๐ท๐ฌ๐ฝ (๏ฟฝ๏ฟฝ๐๐๐,๐ด๐ท๐ฌ๐ฝ ((๐๐๐๐,๐๐๐๐๐1 โ ๐๐๐๐,๐๐๐๐๐ ๐๐๐๐๐๐๐))) (28)
4. Nitrogen Interface Assembly
The NIA is used to pressurize the accumulator as an interface to the Internal Thermal Control System (ITCS). The
rate of exergy destroyed for the accumulator within the NIA is as follows:
แบDES,ACCUMULATOR = ๏ฟฝ๏ฟฝ๐ต๐,๐๐(๐๐2,๐๐ โ ๐๐๐๐๐๐๐๐2,๐๐) โ ๏ฟฝ๏ฟฝ๐ต๐,๐๐๐(๐๐2,๐๐ข๐ก โ ๐๐๐๐๐๐๐๐2,๐๐ข๐ก) (29)
5. Airlock Air Save Pump Package
The airlock air save pump package reduces pressure in the entire airlock from 101.3 to 70.3 kPa for extravehicular
activity (EVA) campout prior to EVAs. The rate of exergy destroyed from operation of the air save pump is as follows:
(30)
D. Exergy Analysis of the Temperature and Humidity Control System
The THC system regulates the temperature and humidity levels within the atmosphere aboard the space station.
Internal factors within the space station such as heat metabolically generated by crewmembers and equipment gener-
ated heat cause temperature to rise. Crew respiration and perspiration allows for humidity levels to increase. The THC
system is set in place to regulate these internal factors. Components included within the THC system include the
Common Cabin Air Assembly (CCAA), the Avionics Air Assembly (AAA), and the intermodule ventilation system
(IMV). The CCAA provides temperature and humidity regulation for the cabin, the AAA provides cooled air and
ventilation needed for the Fire Detection System, and air flow and the removal of CO2 and trace contaminants is
provided by the IMV system to modules that do not have ARS or ACS equipment. Particulate filters rated to high
efficiency particulate air (HEPA) standards are included within the THC system to remove particulate matter and
microorganisms from the space stationโs atmosphere.20
1. Common Cabin Air Assembly
The CCAA consists of a fan assembly, condensing heat exchanger, and water separator for humidity regulation.
The fan assemblyโs exergy rate balance equation is as follows:
(31) แบdes,CCAA Fan = แบ๐ป๐ฏ๐ช ๐ญ๐๐ โ (๐ โ๐ป๐๐๐๐๐
๐ป๐ญ๐๐) ๏ฟฝ๏ฟฝ
๐๐ป๐ถ ๐น๐๐ โ ๏ฟฝ๏ฟฝ๐๐๐,๐๐๐๐๐๐ ((๐๐๐๐,๐ป๐ฏ๐ช ๐ญ๐๐ ๐ถ๐๐ โ
๐๐๐๐๐๐๐๐๐๐,๐ป๐ฏ๐ช ๐ญ๐๐ ๐ถ๐๐) โ (๐๐๐๐,๐ป๐ฏ๐ช ๐ญ๐๐ ๐๐ โ ๐๐๐๐๐๐๐๐๐๐,๐ป๐ฏ๐ช ๐ญ๐๐ ๐๐))
แบ๐ท๐๐ ,๐ด๐๐ ๐๐๐ฃ๐ = ๐ถ๐ท๐ฌ๐ต๐จ๐บ [แบ๐จ๐บ ๐๐๐๐ โ (๐ โ๐ป๐ช๐๐๐๐
๐ป๐ท๐๐๐) ๏ฟฝ๏ฟฝ๐จ๐บ ๐๐๐๐ โ ๏ฟฝ๏ฟฝ๐๐๐,๐๐๐๐๐๐๐ก ๐๐ข๐ก ((๐๐๐๐,๐ด๐ ๐๐ข๐๐ ๐๐ข๐ก
โ ๐๐๐๐๐๐๐๐๐๐,๐ด๐ ๐๐ข๐๐ ๐๐ข๐ก) โ (๐๐๐๐,๐ด๐ ๐๐ข๐๐ ๐๐ โ ๐๐๐๐๐๐๐๐๐๐,๐ด๐ ๐๐ข๐๐ ๐๐))]
American Institute of Aeronautics and Astronautics
10
The condensing heat exchanger is analyzed from an exergy perspective as follows:
(32)
where
๏ฟฝ๏ฟฝ๐๐๐,๐๐๐๐๐๐ = ๏ฟฝ๏ฟฝ๐ ๐๐๐๐๐,๐๐๐๐๐๐ + ๏ฟฝ๏ฟฝ๐๐๐๐๐,๐๐๐๐๐๐ (33)
and
๐ = ๏ฟฝ๏ฟฝ๐๐๐,๐ฏ๐ฟ
๏ฟฝ๏ฟฝ๐๐๐,๐๐๐๐๐๐ (34)
The CCAAโs water separator may be modeled from an exergy perspective as follows:
(35)
where
แนair,WSout = qแนwater,HXout = qkแนwater,fanout (36)
and
แนwater,WSout = (1-q)แนwater,HXout = (1-q)kแนwater,fanout (37)
2. Avionics Air Assembly
The structures of the CCAA and the AAA are relatively similar. The AAA also contains a ventilation fan and a
condensing heat exchanger which have an exergy balance form as follows:
(38)
and
(39)
3. Intermodule Ventilation
The IMV system uses a fan for air recirculation as well. The IMV system may be modeled from an exergy per-
spective as follows:
(40)
4. Particulate Matter Filtration
The particulate filters within the THC are analyzed from an exergy perspective as the changes in deposit mass
kinetic energy as follows:
แบDES,HEPA = ๏ฟฝ๏ฟฝ๐๐๐,๐ฏ๐ฌ๐ท๐จ ( ๐ฝ๐ฏ๐ฌ๐ท๐จ,๐๐๐
๐
๐โ
๐ฝ๐ฏ๐ฌ๐ท๐จ,๐๐๐
๐) โ ๏ฟฝ๏ฟฝ๐ ๐๐๐๐๐๐ ,๐ฏ๐ฌ๐ท๐จ (
๐ฝ๐ฏ๐ฌ๐ท๐จ,๐๐๐
๐) (41)
E. Exergy Analysis of the Water Recovery and Management Subsystem
The purposes served by the WRM subsystem are to distribute potable water for crew utilization, and to collect
wastewater for processing. The system effectively monitors water quality, generates a potable water supply for both
consumption and for hygiene purposes, and processes wastewater and urine. Wastewater consists of condensate from
the THC system, hygiene return water, pretreated urine, and wastewater from the Extravehicular Mobility Unit.20 For
modelling the WRM subsystem from an exergy perspective, we analyze the urine processor assembly, water processor,
แบdes,THC WS = แบ๐พ๐บ ๐๐๐๐ + แบ๐พ๐บ ๐๐๐๐ โ (๐ โ๐ป๐๐๐๐๐
๐ป๐๐๐๐)๏ฟฝ๏ฟฝ๐พ๐บ ๐๐๐๐ + ๐๏ฟฝ๏ฟฝ๐๐๐๐๐,๐๐๐๐๐๐(๐๐ค๐๐ก๐๐,๐ป๐ฏ๐ช ๐พ๐บ ๐๐ โ
๐๐๐๐๐๐๐๐ค๐๐ก๐๐,๐ป๐ฏ๐ช ๐พ๐บ ๐๐) โ ๏ฟฝ๏ฟฝ๐๐๐๐๐,๐พ๐บ ๐๐๐(๐๐ค๐๐ก๐๐,๐ป๐ฏ๐ช ๐พ๐บ ๐๐๐ โ ๐๐๐๐๐๐๐๐ค๐๐ก๐๐,๐ป๐ฏ๐ช ๐พ๐บ ๐๐๐) โ
๏ฟฝ๏ฟฝ๐๐๐,๐พ๐บ ๐๐๐(๐๐๐๐,๐พ๐บ ๐๐๐ โ ๐๐๐๐๐๐๐๐๐๐,๐พ๐บ ๐๐๐)
แบdes,IMV Fan = แบ๐ฐ๐ด๐ฝ ๐ญ๐๐ โ (๐ โ๐ป๐๐๐๐๐
๐ป๐ญ๐๐) ๏ฟฝ๏ฟฝ๐ผ๐๐ ๐น๐๐ โ ๏ฟฝ๏ฟฝ๐๐๐,๐ฐ๐ด๐ฝ ๐๐๐๐๐๐ ((๐๐๐๐,๐ฐ๐ด๐ฝ ๐ญ๐๐ ๐ถ๐๐ โ
๐๐๐๐๐๐๐๐๐๐,๐ฐ๐ด๐ฝ ๐ญ๐๐ ๐ถ๐๐) โ (๐๐๐๐,๐ฐ๐ด๐ฝ ๐ญ๐๐ ๐๐ โ ๐๐๐๐๐๐๐๐๐๐,๐ฐ๐ด๐ฝ ๐ญ๐๐ ๐๐))
แบdes,AAA Fan = แบ๐จ๐จ๐จ ๐ญ๐๐ โ (๐ โ๐ป๐๐๐๐๐
๐ป๐ญ๐๐) ๏ฟฝ๏ฟฝ๐ด๐ด๐ด ๐น๐๐ โ ๏ฟฝ๏ฟฝ๐๐๐,๐จ๐จ๐จ ๐๐๐๐๐๐ ((๐๐๐๐,๐จ๐จ๐จ ๐ญ๐๐ ๐ถ๐๐ โ
๐๐๐๐๐๐๐๐๐๐,๐จ๐จ๐จ ๐ญ๐๐ ๐ถ๐๐) โ (๐๐๐๐,๐จ๐จ๐จ ๐ญ๐๐ ๐๐ โ ๐๐๐๐๐๐๐๐๐๐,๐จ๐จ๐จ ๐ญ๐๐ ๐๐))
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condensate storage assembly, fuel-cell water storage tanks, wastewater vent assembly, electrical work performed by
the system, and water distribution network. Equation 41 describes the rate at which exergy is destroyed within the
WRM subsystem.
แบDES,WRM =
(42)
F. Exergy Analysis of the Waste Management Subsystem
The Waste Management (WM) subsystem consists of commode and urinal, blower, piston, plenum filter, and
output filter. Cabin air is pulled through the commode seat to draw in feces through the waste canister. The blower
engages once the seat is lifted and operates for about thirty seconds after the seat lid is closed. The piston operates to
compress feces collected in a plastic collection bag placed in the waste canister. Canisters are replaced regularly, and
a filter lid is placed on the canister. Canisters are then stored for a return to Earth.20 The rate of Exergy destroyed
within the urinal and commode may be modeled as follows:
(43)
The waste management blower may be modeled from an exergy perspective as follows:
(44)
The rate of exergy destroyed by the waste management piston may be modeled as follows:
(45)
The rate of exergy destroyed by the waste management plenum filter may be modeled as follows:
๏ฟฝ๏ฟฝ๐ท๐ธ๐,๐๐ ๐ต๐๐๐ค๐๐ = ๏ฟฝ๏ฟฝ๐๐ ๐ต๐๐๐ค๐๐ โ (1 โ๐๐ถ๐๐๐๐
๐๐ต๐๐๐ค๐๐) ๏ฟฝ๏ฟฝ๐๐ ๐ต๐๐๐ค๐๐
โ ๏ฟฝ๏ฟฝ๐๐๐,๐๐ ๐ต๐๐๐ค๐๐ ๐๐ข๐ก ((โ๐๐๐,๐๐ ๐ต๐๐๐ค๐๐ โ ๐๐ถ๐๐๐๐๐ ๐๐๐,๐๐ ๐ต๐๐๐ค๐๐ ๐๐ข๐ก) โ (โ๐๐๐,๐๐ ๐ต๐๐๐ค๐๐ ๐๐ โ ๐๐๐๐๐๐๐ ๐๐๐,๐๐ ๐ต๐๐๐ค๐๐ ๐๐))
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(46)
The rate of exergy destroyed by the waste management output filter may be modeled as follows:
(47)
V. Discussion and Conclusion
The exergy balance equations developed herein illustrate the universal application of exergy to a diversity of sys-
tems. Systems ranging from waste management to oxygen generation are able to be modeled to determine their exergy
efficiency. With the correct inputs these equations can provide both system and subsystem efficiencies. This type of
analysis enables the system designer to identify areas of concern and where to allocate resources. Future work will
examine multiple versions of the ECLS system to determine the efficiencies of the designs. Furthermore the current
system will be analyzed using the equations formed in this paper to identify areas for improvement, a critical step in
advancing the ECLS system for interplanetary transport.
Acknowledgments
Funding for this research was provided by the NASA Systems Engineering Research Consortium.
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